1. Field of the Invention
The present invention relates to systems and methods of water purification, and, more particularly, the control of nutrients, suspended and filamentous algae, pollutants, and toxins in water.
2. Description of Related Art
Many freshwater lakes and ponds, as well as estuaries, are characterized, particularly during the warmer months, by a significant population of suspended algae or phytoplankton in the water body""s water column. These largely unicellular plants give the water a greenish and often a xe2x80x9cpea-soupxe2x80x9d appearance that many observers find unattractive. Floating mats of unsightly, filamentous algae also can occur. High concentrations of algae may lead to low levels of dissolved oxygen in the early morning hours, leading to stress on the aquatic and fish populations. In extreme cases, these conditions will lead to fish kills and the general decline of the quality of a water body.
The basis of the problem is an abundance of soluble nutrients within the water body, which then allows the rapid growth and maintenance of the elevated population of suspended or filamentous algae. The source of the soluble nutrients may be sediments, air deposition, point source polluting discharges, generalized, non-point-source inflows, or most likely a combination of all these factors. An effective management strategy would combine elements of attempting to reduce nutrient loading to the water body with treatment of the water body itself.
Currently used methods of controlling algal growth in ponds or lakes typically involve treating the water with selected herbicides or xe2x80x9calgicides.xe2x80x9d These chemicals kill the suspended algae, returning the water to its more desired appearance of clear or only slightly colored waters. Alternatively, a water body may be treated with various aluminum salts (e.g., aluminum sulfate), which achieves a similar result through a chemical precipitation reaction. Another strategy employed in managing algae is to introduce a dye that then, via the mechanism of shading, achieves the same result of killing the algae and returning the water to its algae-free appearance.
Another problem with these approaches is that the underlying feature that initially encouraged the growth of the algae remains; that is, the nutrients on which the algae grew remain in the water, and after the effects of the algicide, herbicide, or dye decrease, the conditions for a renewed growth of algae are abundantly present. In addition, for the algicide and herbicide at least, the negative environmental effects of potentially toxic accumulation must be considered. In the case of aluminum salts, a temporary reduction in selected nutrients (e.g., phosphorus) is effected, but considerable skill and expertise are required to effectively and efficiently precipitate the suspended (nutrient-containing) solids.
An alternative strategy to killing the algae and then creating relatively clear but nutrient-rich water body is to cause a release of nutrients from the algae but then to remove these nutrients from the water body. A natural method of achieving this nutrient removal is through the harvesting of macrophyte vegetation, which takes up the soluble nutrients as a function of their growth. If the total mass of nutrients removed through plant harvest were to match the ongoing nutrient loading through the various sources of sediment transport, air deposition point and nonpoint sources, then the lake or pond would be able to maintain an algal-free appearance.
Shading of the suspended algae-containing water is one means to achieve algal cell lysis and an increase in the proportion of nitrogen and phosphorus in the water column that is biologically available for other plant growth. The use of shading to cause a release of soluble nutrients has been investigated and taken advantage of in the prior art.
Reddy and DeBusk (1987) determined in a short-term mesocosm-scale experiment that the primary nutrient removal mechanism in a water hyacinth system that received phytoplankton-laden lake water was the settling of algal cells.
The sustainable removal of nutrients, however, involves not just shading and subsequent plant uptake and plant harvest. Many macrophytes, such as the floating water hyacinths orsubmerged macrophytes, are characterized by much highergrowth rates than can be adequately sustained by the amount of nutrients held by a water column beneath them. Reddy et al. (1983) concluded that the high levels of floating water hyacinth biomass in a central Florida lake could be sustained only by transport of nutrients from the sediments and detritus or by fixation (in the case of nitrogen) from the atmosphere.
A particular model of a system for decreasing algal concentration consists of macrophyte vegetation (plants) and various permutations of floating boom, barrier, and water control mechanisms (pumps, internal barriers, etc.) for sequentially shading suspended algae-containing water. The shading causes the algae to lyse and release contained nutrients, which then promotes the growth of a standing crop of either subsurface (submerged) or floating vegetation. Depending on the configuration desired, the plants are periodically harvested from the containment system, effectively removing soluble nutrients such as phosphorus and nitrogen from the pond, estuary, or lake. In the case of submerged macrophytes, water chemistry changes caused by their photosynthetic activity can further contribute to nutrient removal (e.g., co-precipitation of phosphorus with calcium carbonate) beyond that achieved by plant harvest. As the overall total level of nutrients within the water column decreases, the conditions favorable to sustained nuisance algal growth diminish, and further algal growth is discouraged.
Therefore, to encourage further plant growth and hence continued removal of suspended and filamentous algae, it is believed desirable that the water beneath the macrophyte vegetation, whether held within a barrier or not, be exchanged with new nutrient-bearing water.
Another site of contaminated water is the so-called xe2x80x9cwaste stabilization pondxe2x80x9d (WSP), a body of water used to store industrial, municipal, agricultural wastewater or contaminated groundwater. The WSP is believed to be the most prevalent type of wastewater treatment technology in the world.
The system and method of the present invention is directed to the management and control of suspended and filamentous algae, excess nutrients, hydrocarbons, pathogens, and other contaminants in fresh water bodies and estuaries, including natural and manmade bodies of water such as waste stabilization ponds. A plurality of embodiments are contemplated, a best mode among which is dependent upon the characteristics of each specific body of water, as determinable by measurements of water chemistry, algal decomposition rates, and contaminant type and level.
A method of the present invention for reducing a contaminant level in a body of water comprises the steps of placing plants atop at least a portion of the body of water. The plants are positioned to shield the water column from sunlight sufficiently to kill phytoplankton therein, the lysis of which releases nutrients, at least a portion of which is sequestered by the plants. A microbial colonization of a rhizosphere area (root zone) of the plants is permitted for achieving microbially mediated contaminant removal and transformation. The microbes can work to degrade hydrocarbons, and water-borne pathogens may be consumed by root-zone biota, for example. In addition, the microbial population may mediate a transformation of ammonia nitrogen to nitrate nitrogen to nitrogen gas.
In a system for reducing a contaminant level, the plants comprise a floating mat of plants placed atop at least a portion of the body of water. The floating mat serves two functions: It alters the underlying water column environment by shading; and it provides a high-surface-area attachment site, the roots and root hairs, for microbes to colonize. It is believed that the rhizosphere can harbor a complex array of microorganisms that proliferate on attachment sites and rely on inorganic and organic exudates (e.g., organic compounds and enzymes) from the plant roots to support their physiological processes. The present inventors have shown, for example, that for some aquatic plant species there exists significant oxygen transfer from the plant shoots to the roots, thereby facilitating aerobic microbial respiration in the root zone.
Another embodiment is a method comprising the steps of segregating a column of water within a body of water having excess phytoplankton and nutrients therein and shielding the water column from sunlight sufficiently to kill phytoplankton therein, the lysis thereof releasing nutrients. Next the released nutrients are sequestered, and remediated water is replaced in the water column with water from the body of water.
In some embodiments, floating or submerged macrophytes are introduced into a body of water in combination with water column shading and induced water movement to optimize nutrient uptake by the macrophytes. Water column shading may also be achieved by the macrophytes themselves. The macrophytes can include aquatic plants that float on their own, such as water hyacinths, submerged aquatic plants, and terrestrial or aquatic plants caused to float by artificial means.
Another developing problem in bodies of fresh water is the presence of toxic algae. At present copper is added to the water to kill the toxic algae, but the difficulty is that this releases the toxins into the water.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.